(1) Field of the Invention
This invention relates to a transmission apparatus and, more particularly, to a transmission apparatus for multiplexing and transmitting signal lights.
(2) Description of the Related Art
One of methods for realizing a large-capacity optical communication system is a communication method employing Wavelength Division Multiplexing (WDM) technology. This WDM technology is expected to offer flexible system operation and reliable circuit quality, for example, to cope with a system which is dynamically established by changing the number of operating wavelengths (the number of signal lights to be input to the system) or switching an optical path.
Some optical amplifiers that are used in the WDM technology operate in two operation modes: Automatic Gain Control (AGC) mode and Automatic Level Control (ALC) mode. In AGC mode, the optical amplifiers maintain their constant gains over signal lights of certain input wavelengths. In the ALC mode, the optical amplifiers maintain their output power according to the number of operating wavelengths. The optical amplifiers generally operate in ALC mode, and if the number of operating wavelengths varies, the ALC mode is switched to the AGC mode. As a result, stable optical communication can be performed even if the number of wavelengths changes (for example, refer to Japanese Unexamined Patent Publications Nos. 2000-236301 and 8-195733).
FIG. 8 is a block diagram of a conventional optical communication system. As shown in this figure, this optical communication system comprises a transmitting station 110, a repeater 120, and a receiving station 130, for example. The transmitting station 110 combines the wavelengths of signal lights from optical senders (which are represented by OS in the figure) 101 to 103, and amplifies and outputs the wavelength-multiplexed signal light to the repeater 120. The repeater 120 amplifies the received signal light to compensate for attenuation caused through a transmission path from the transmitting station 110, and outputs the resultant to the receiving station 130. The receiving station 130 amplifies the received signal light to compensate for attenuation caused though a transmission path from the repeater 120, separates the resulting signal light by wavelength, and outputs them to optical receivers (which are represented by OR in the figure) 141 to 143.
The transmitting station 110 has an optical multiplexer 111, couplers 112 and 116, a spectrum analyzer 113, an optical amplifier 114, and a monitoring/control unit 115. Signal lights output from the optical senders 101 to 103 enter the optical multiplexer 111. The optical multiplexer 111 then combines the wavelengths of the signal lights and outputs the wavelength-multiplexed signal light to the spectrum analyzer 113 and the optical amplifier 114 via the coupler 112.
The spectrum analyzer 113 separates by wavelength the wavelength-multiplexed signal light, which is output from the optical multiplexer 111, and calculates optical power for each wavelength. Then the spectrum analyzer 113 calculates the number of operating wavelengths (the number of signal lights existing in the transmitting station 110) based on the detected wavelengths and their optical power. The spectrum analyzer 113 gives the calculated number of operating wavelengths to the optical amplifier 114 and the monitoring/control unit 115.
The optical amplifier 114 processes the signal light in AGC or ALC mode, depending on the number of operating wavelengths given from the spectrum analyzer 113. The optical amplifier 114 generally operates in ALC mode so as to output signal light with constant optical power according to the number of operating wavelengths. If the number of operating wavelengths changes due to addition or removal of optical senders 101 to 103, the optical amplifier 114 operates in AGC mode. In addition, the optical amplifier 114 also operates in AGC mode if the number of operating wavelengths for multiplexed signal light output from the optical multiplexer 111 varies due to a fault occurring in the optical multiplexer 111.
The monitoring/control unit 115 transmits a monitoring control signal OSC including the number of operating wavelengths to the repeater 120 and the receiving station 130, the number of operating wavelengths given from the spectrum analyzer 113. The repeater 120 and the receiving station 130 should be notified of the number of operating wavelengths because they do not have a spectrum analyzer. In addition, the monitoring/control unit 115 communicates with the optical amplifier 114 to monitor and control operations of the optical amplifier 114.
The repeater 120 has couplers 121 and 124, an optical amplifier 122 and a monitoring/control unit 123. The signal light output from the transmitting station 110 enters the optical amplifier 122 via the coupler 121. The monitoring control signal OSC output from the transmitting station 110 enters the monitoring/control unit 123 via the coupler 121. The monitoring/control unit 123 extracts the number of operating wavelengths from the monitoring control signal OSC, and gives it to the optical amplifier 122 and the coupler 124. The optical amplifier 122 processes the signal light in AGC or ALC mode, depending on the number of operating wavelengths given from the monitoring/control unit 123. Note that the optical amplifier 122 switches between the AGC mode and the ALC mode under the same conditions as the optical amplifier 114.
The receiving station 130 has a coupler 131, an optical amplifier 132, a monitoring/control unit 133 and an optical demultiplexer 134. The signal light output from the repeater 120 enters the optical amplifier 132 via the coupler 131. The monitoring control signal OSC output from the repeater 120 enters the monitoring/control unit 133 via the coupler 131. The monitoring/control unit 133 extracts the number of operating wavelengths from the monitoring control signal OSC and gives it to the optical amplifier 132. The optical amplifier 132 processes the signal light in AGC or ALC mode, depending on the number of operating wavelengths given from the monitoring/control unit 133. Note that the optical amplifier 132 switches between the AGC mode and the ALC mode under the same conditions as the optical amplifier 114. The optical demultiplexer 134 separates by wavelength the signal light received from the optical amplifier 132, and outputs them to the optical receivers 141 to 143.
Cost for establishing an optical communication system is a key for Metro, etc. Therefore, there is an optical communication system which realizes optical communication without a spectrum analyzer 113 that is expensive.
FIG. 9 is a block diagram of another conventional optical communication system without a spectrum analyzer. A transmitting station 150 of FIG. 9 does not have a coupler 112 and a spectrum analyzer 113 that the transmitting station 110 of FIG. 8 has, and has couplers 151 to 153 and a power sensor 154. Identical components in FIG. 8 and FIG. 9 have the same reference numerals.
In the optical communication system of FIG. 9, signal lights at the input stage of the optical multiplexer 111 enter the power sensor 154 comprising a photo diode (PD), via the couplers 151 to 153. The power sensor 154 detects the number of signal lights having prescribed optical power or higher, to thereby detect the number of operating wavelengths. The optical amplifier 114 switches between the AGC mode and the ALC mode depending on the number of operating wavelengths detected by the power sensor 154. That is to say, this system does not require a spectrum analyzer 113, thus realizing a simple construction at a low cost.
This technique of obtaining the number of operating wavelengths at the input stage of the optical multiplexer, however, has a drawback in that, since faults which occur between the input stage of the optical multiplexer and the input stage of the optical amplifier are not monitored, the operation modes cannot be switched appropriately for the faults.
For example, in FIG. 9, assume now that signal lights are output from optical senders 101 to 103 and three operating wavelengths are detected by the power sensor 154. In this situation, if a signal path of the optical multiplexer 111 to the optical sender 101 is disconnected and signal light over two wavelengths combined is output from the optical multiplexer 111, the number of wavelengths varies from three to two. In this case, the optical amplifier 114 should operate in AGC mode.
Since the spectrum analyzer 113 separates by wavelength signal light at the input stage of the optical amplifier 114 in order to obtain the number of operating wavelengths, this analyzer 113 can detect the change in the number of wavelengths due to the fault occurring in the optical multiplexer 111. The power sensor 154, however, cannot recognize the change in the number of wavelengths due to the fault occurring in the optical multiplexer 111, so that the optical amplifier 114 cannot switch the operation mode.